This invention pertains to the art of separating a liquid into fractions by pressure-driven flow through semi-permeable membranes, known as ultrafiltration, and more particularly, to laboratory apparatus adapted to accomplish that task on a small scale.
Ultrafiltration is the general term applied to the process of separating a liquid into fractions by pressure-driven flow through semi-permeable membranes. By proper selection of the membrane material, it is possible to separate liquids based upon their molecular weight, thus obtaining a permeate of extremely high purity. Such processes find wide application in a number of industries, as for separating milk used in cheese making into whey and milk precheese product, and in concentrating antibiotics from a fermentation broth.
Two distinctions are important in identifying the position of the present invention in relation to the art. First, the art classifies filtration processes into microfiltration, ultrafiltraton, and hyperfiltration (or reverse osmosis). The distinction between these processes is based primarily upon the pore size of the membranes employed and the pressure at which the systems operate. Microfiltration operates at a relatively large pore size (0.02-2.0 micron) and low pressure (30-150 psi). Hyperfiltration, or reverse osmosis, operates at pore sizes from about 5-15 angstroms down to the micromolecular and ionic size range (molecular weights of 150 and below), and at pressures in the range 200-1000 psi. Ultrafiltration operates at intermediate values, with molecular weight cutoffs ranging from 200 to 350,000 and pore diameters of from about 10 to 1000 angstroms. Although the preferred embodiment of the present invention is directed primarily toward ultrafiltration and microfiltration, the invention would operate equally well in a microfiltration role, and it could be adapted to hyperfiltration equipment as well.
Second, the field of ultrafiltration encompasses several means of effecting the separation of a liquid into fractions. At the outset, it should be understood that ultrafiltration does not operate in a manner analogous to "filtering" processes, in which a liquid is passed through a filter disposed transverse to the flow path, with undesirable solids being retained by the filter and the objective being a clarified liquid output. Rather, ultrafiltration seeks to separate a base liquid into two fractions by placing the liquid in the presence of a semi-permeable membrane; one portion of the liquid (termed the permeate) will pass through the membrane, and the other will remain in the base liquid stream, termed the retentate. Thus, ultrafiltration systems pass a base liquid across, rather than through, the filtration means. Also, depending upon the specific application, one fraction or the other may be the desired product of the process. For example, in cheese manufacture, the desired product is the retentate (precheese liquid), while in a juice manufacturing process the object is the permeate, a clarified fruit juice.
Several methods of ultrafiltration have been suggested by the art. Of primary concern to the present invention are the methods classified as "plate and frame" ultrafiltration, in which a series of plates support semi-permeable membranes, and the base liquid is passed across these membranes for filtration. Other methods include spiral membrane apparatus, in which the membrane is wrapped in a perforate collection tube, the base liquid being passed through the tube longitudinally. A membrane also may be presented in tubular form, with the base liquid passed within the tube and the permeate passing through the tube and collecting within the membrane housing. Alternatively, hollow fiber membranes have been offered, with a bundle of hollow fiber membranes contained within a tubular housing. Base liquid is passed though the cores of the fibers, and permeate is collected from the channels surrounding the fibers. The disadvantages of these methods, when compared to the present invention, will be clear to those skilled in the art.
The preferred plate-and-frame processes depend, of course, on the presentation of a large membrane area to the base liquid, and it is known in the art to employ membrane supports, with membrane material disposed on both sides of a plate and the plates arranged in a stack. Generally, such a stack is provided with input and output flow passages for the base liquid, disposed on opposite sides of the stack such that liquid can flow to one side of a plate and thence between the membranes of adjacent plates, allowing the base liquid to come into intimate contact, under pressure, with the membrane surface to permit ultrafiltration. Because one pass through the system generally does not suffice to provide complete extraction of the desired constituents, the retentate usually is recirculated through the ultrafiltration apparatus several times. Further, it is known to divide the stack into subassemblies, each subassembly having input and output passages, such that liquid flows in parallel across the membranes of all support members of a subassembly, and the output of one subassembly flows to the input of a succeeding subassembly.
The apparatus available to date has exhibited a number of problems. Ultrafiltration equipment is evaluated based on two criteria--the concentration ratio, reflecting the maximum concentration to which the base liquid can be processed (defined as the ratio of initial volume of base liquid to the final volume after processing), and the flux rate, defined as the volume of permeate that passes through a given area of membrane per unit of time, generally expressed as gallons of permeate per square foot of membrane per day (GFD) or liters of permeate per square meter of membrane per hour. These two factors will determine the specifications of an ultrafiltration apparatus chosen for a particular application.
Typical of the apparatus offered by the art is the ellipsoidal structure seen in U.S. Pat. No. 3,872,015, issued to Madsen. As disclosed, the apparatus is similar to that discussed above, with each plate-like member being ellipsoidal in form. Each plate also has two openings formed toward the ends of the major axis, so that when the stack is formed by passing retaining bolts through the aligned openings, inlet and outlet passages are formed. Curved grooves in the surface of the plate extend from one opening to the other. These grooves generally can be described as forming a set of concentric ellipses of increasingly smaller periphery. Blocking members placed in one opening of periodically-spaced members serve to divide the stack into subassemblies, as discussed.
Base liquid flows through the inlet passage of a subassembly and passes into the gap between adjacent membranes. The fluid pressure of the base liquid forces both membranes against the respective plate surfaces, so that liquid flows within channels corresponding to the surface grooves, and therein lies the fundamental flaw with this design. Given that the fluid pressure at the head of all channels is equal and that the flow path in the outer channels is significantly longer than that of the inner channels, basic principles of fluid dynamics would lead one to expect the flow velocity in the inner channels to be significantly greater than in the outer channels. That expectation is borne out in operation. As the viscosity of the retentate increases, fluid velocity in the outer channels decreases, ultimately dropping to zero. Before that point, however, the solids entrained in the liquid are deposited, and the channel rapidly plugs. The relatively short inner channels in effect "short-circuit" the flow pattern, and this process continues as the base liquid becomes more concentrated with repeated recirculation through the system.
The assignee of this patent has attempted to alleviate this problem by eliminating the central portion of the plate, leaving an ellipsoidal ring, and by increasing the depth of the outer (longer) channels. That design does ameliorate the plugging problem, but at the expense of reduced output (from reduced membrane area) and higher cost (from inefficient production of membrane material--the cutout central section cannot be put to other use). Moreover, observation of this design reveals that the uneven flow rate leads to uncertainty as to which channels will plug first, as sometimes an inner channel plugs, and at other times an outer channel will become blocked. The problems with this design stem directly from the provision of flow channels of uneven length, and appear inherent in such configurations. Clearly, this approach does not meet the needs of the antibiotic industry, or indeed the needs of any application requiring ultrafiltration of base liquids over a wide viscosity range.
An alternative approach is disclosed in U.S. Pat. No. Re. 30,632 (a reissue of U.S. Pat. No. 3,831,763), to Breysse. The basic structure of this device is similar to that discussed above, but here the plates are rectangular, and joining members are disposed between adjacent plates to promote sealing and to define the space into which the base liquid flows between plates. Each plate has two openings, defining inlet and outlet passages, and intermediate plates, having only one such opening, serve to divide the stack into subassemblies. A depression is formed into the surface of both sides of a plate, and packing material is carried therein to permit collection of permeate, and the membranes are carried atop this material. A variant form of this device, offered by the assignee of this patent, substitutes raised ridges, formed in the surface of the depression and extending across same.
Two problems have arisen in the application of this device to fields requiring operation over wide viscosity ranges. First, the inclusion of joining members (typically, gaskets) at the outer periphery of each plate severely limits the pressure at which the base liquid can be introduced into the inlet passages. Of course, such a limitation restricts the overall flow rate and the resultant output.
More seriously, this design again does not prevent deposition of solids from the base liquid, particularly in antibiotic applications. As with the previous device, the problem stems from the basic principles of fluid dynamics. It is well known that fluid flow within a channel is not uniform but exhibits a velocity profile from one side of the channel to the other. Velocity is lowest at the sides of the channel (indeed, it is zero within a boundary layer in contact with the channel wall). Further, the velocity differential across the channel is related to the viscosity of the fluid (higher viscosity produces a higher differential) and to the size of the channel (wider channels result in a more pronounced zone of significantly lower velocity). These theoretical predictions again are seen to occur in practice. When employed in an antibiotic application, where the base fluid contains a high level of suspended solids, flow velocity of the broth at the edges of this device is not sufficiently high to avoid deposition, restricting the flow to an increasingly small area toward the center of each plate. Output suffers, both from the reduced permeate flow and from the increased requirement to clean and change membranes.
A common shortcoming of these devices is the failure to provide uniform flow across the surface of each plate, at flow rates that offer economically-attractive permeate recovery. Rather, it was left to the inventors of the present invention to devise apparatus that functions over the viscosity ranges required by the antibiotic and other industries.
These problems ar accentuated when dealing with apparatus scaled for laboratory use rather than for full-scale production. The low flow rates of such apparatus exacerbate the difficulties presented by solids deposition discussed above. Although both the ellipsoidal and flat-plate devices noted above have been adapted to laboratory use, both such devices display the same disadvantages as seen in their full-scale embodiments. What is missing from the laboratory is a small-scale unit that provides a high permeate yield, coupled with the ability to handle a wide range of viscosities. The present invention provides such a device.